Diffusion induced grain-boundary migration in alumina and its effect on mechanical properties

Abstract

Alumina ceramics are widely used as ceramic components in modern electronic and machine industries, for example IC boards, bearings, cutting tools, etc. However, the applications of the ceramic materials including alumina have been restricted because of their susceptibility to brittle fracture. Brittle fracture may catastrophically occur from the surface flaws, bringing about the poor reliability of the components. The Diffusion Induced Grain-Boundary Migration (DIGM) can be a method for the modification of surface microstructure of alumina ceramics. In the present thesis, we studied the diffusion-induced microstructure changes and then their effects on mechanical properties. The evaluation of mechanical properties of surface modified or layer-structured materials has been investigated. In Chap. Ⅱ, the diffusion induced surface undulation was studied in polycrystalline and single crystal alumina using $Fe_2O_3$ and $Cr_2O_3$. When the alumina sample are annealing in a chemical inequilibrium condition, undulation of solid-vapor interface as well as the DIGM occurs. The undulated solid-vapor interfaces were investigated using scanning electron microscopy and cross-sectional transmission electron microscopy. Much more misfit dislocations were observed beneath the protrusion regions than at the concave regions. The driving force for such a material transfer, which resulted in an undulation of the surface, was explained in terms of the diffusional coherency strain. In Chap. Ⅲ, surface modified alumina has been designed by using the DIGM technique in 1000 ppm Fe-doped alumina. To minimize the chemical effect on mechanical properties, only 1000 ppm Fe (Fe/Al) was doped. 1000 ppm Fe-doped alumina samples were sintered at 1500℃ for 3 h in $95N_2-5H_2$. Sintered sample had a typical polycrystalline microstructure with average grain size of about 6 mm. Transmission electron microscopy (TEM) observation showed that excess $Fe_2O_3$ remained as a-Fe...